(a) Field of the Invention
This invention relates to a releasable heat-sensitive recording paper and more particularly to a releasable heat-sensitive recording paper usable with effect especially as a label wherein a heat-sensitive color forming layer and a protective layer are laminated on a substrate, a required pattern is printed on this protective layer with an ultraviolet setting-type printing ink, a pressure-sensitive adhesive layer is formed on the back of the substrate and a releasable paper is provided so as to cover said adhesive layer.
(b) Description of the Prior Art
Labels have hitherto been used everywhere, and many labels have been employed in shops especially on articles which bear labels indicating necessary values such as weights, prices, and the like. These labels have usually been prepared by print processes or ribbon processes (such as stamping press and the like). However, such processes, where an oily or water-based printing ink is used, which ink is inferior in drying ability, involve defects such that much time is consumed for drying and blocking is liable to occur. In addition, ribbon processes are disadvantageous in that much ribbon material is unavoidably wasted and consequently the production cost is increased.
Various attempts have been proposed to prepare labels by using releasable heat-sensitive recording papers. Heat recording processes using releasable heat-sensitive recording papers (for instance, the printing process using a thermal head printer) are surely advantageous in that maintenance is almost dispensed with, the apparatus can be made compact and further the printer itself is not expensive. However, it is to be noted that the releasable heat-sensitive recording papers which have been devised and proposed up to now are each prepared by forming a heat-sensitive color forming layer on a substrate and further printing a required pattern on said heat-sensitive color forming layer, while forming a pressure-sensitive adhesive layer (tacky layer) on the back of the substrate and covering it with a releasable paper, and are arranged so that the area other than said printed area is first allowed to form color and then the releasable paper is removed from the adhesive layer so that the recording paper may be put on the object to be labeled.
It is well known in this art that the heat-sensitive color forming layer of this releasable heat-sensitive recording paper comprises a dispersion of a colorless or pale leuco dye having a lactone, lactam or spiropiran ring and an acidic substance (for instance, such as an organic acid, phenolic substance or the like) in a binder, which can exhibit a more clear-cut color tone than other heat-sensitive color forming materials and further can achieve the desirable effect that it is free from fog.
Nevertheless, the fact is that labels prepared by using conventional releasable heat-sensitive recording paper are not recognized to have practical value. The reason is that such labels are very often used in the market in such a manner that they are stuck to plastic sheets (or bags made of plastics) and said label-bearing bags are piled up. The color image formed on the label fades away by the action of a plasticizer, for instance, such as dioctyl adipate, dioctyl phthalate or the like, incorporated in the plastic sheet put directly on the label. The phenomenon that this colored image fades away by the action of a plasticizer has not yet been explained. However, it is conjectured that said phenomenon is caused because the colored dye is transformed to the original leuco dye by a certain action exerted thereon from the outside (for instance, it comes in contact with fingers) or by the action of a plasticizer. In this connection, it is noted that the influence to be exerted by the plasticizer incorporated in the label-bearing plastic sheet is faint to a negligible extent because said influence is prevented by the adhesive layer of the label.
In the conventional thermal recording papers, furthermore, the above-mentioned blocking problem remains unsolved because when a required pattern is printed, with a water or oily printing ink, on the area other than the colored image area, it is feared that color formation will be caused in the background because the heat-sensitive color forming layer is heated in order to dry said printing ink rapidly.